*{
    margin: 0;
    padding: 0;
}
.box{
    display: flex;
    justify-content: center;
    align-items: center;
}
.boxA{
    width: 400px;
    height: 400px;
    background-color: red;
    .boxB{
        width: 300px;
        height: 300px;
        background-color: green;
        .boxC{
            width: 200px;
            height: 200px;
            background-color: khaki;
        }
    }
}